BOSE-CONDENSATION OF THE LOCALIZED PAIRS IN 2-BAND SUPERCONDUCTORS

Superconductor with two singularities inherent in high-T-c materials, low carrier density and overlapping of the energy bands on the Fermi s urface, is considered. Provided T=0, the order parameter Delta(n) and chemical potential mu are determined in the mean-field approximation f or the state with Bose condensation of the localized pairs (mu<0). The equation for the energy of the bound state epsilon(b) is also obtaine d and the relationship epsilon(b)=2 mu is established. The method of f unctional integration concerning the two-band model is developed. On t he basis of this method, the crossover between the Fermi picture and t he Bose one of the elementary excitations is demonstrated for the syst em in the presence of the two-particle bound state. The expression for the temperature of Bose condensation T-k is Obtained and the contribu tion of the residual interaction between bosons for the systems with a rbitrary dimension is determined.